Process of making variable wall thickness tubing

ABSTRACT

A method for manufacturing a tube with varying wall thickness includes the step of drawing a tube over a generally cylindrical mandrel having an outer diameter that fluctuates along the length of the mandrel between a first diameter and a second diameter. The tube is drawn over the mandrel to form a tube wall having a thickness that fluctuates along the length of the tube between a first wall thickness and a second wall thickness. After the tube is drawn over the mandrel, the mandrel is removed from the tube. The outer surface of the tube may be finished to form a substantially uniform outer diameter along the length of the tube.

FIELD OF THE INVENTION

The present invention relates to methods for manufacturing smalldiameter tubes, and more specifically to methods of forming smalldiameter tubes having varying wall thicknesses.

BACKGROUND

In the present state of the art, small diameter tubes and hoses aremanufactured for use in a variety of industrial and commercialapplications, including air conditioners and radiators. Many of thesetubes are manufactured with a varying inner diameter to achieve certainproperties, including tube flexibility. For example, some manufacturingmethods form tubes with an annular profile having multiple corrugationsimpressed into the tube. Other manufacturing methods form tubes with asingle helical corrugation along the length of the tube. Thesemanufacturing methods are capable of varying the inner diameter oftubes, but also create a varying outer diameter on the tube. Inaddition, these methods typically form tubes with a relatively constantwall thickness. Therefore, present methods for forming tubes leave muchto be desired, especially in those applications that require tubeshaving constant outer diameters and variable wall thicknesses.

SUMMARY OF THE INVENTION

A method for manufacturing a variable wall thickness tube includes thestep of drawing a tube over a generally cylindrical mandrel having anouter diameter that fluctuates along the length of the mandrel between afirst diameter and a second diameter. The tube is drawn over the mandrelto form a tube wall having a thickness that fluctuates along the lengthof the tube between a first wall thickness and a second wall thickness.After the tube is drawn over the mandrel, the mandrel is removed fromthe tube. The outer surface of the tube may be finished to form asubstantially uniform outer diameter along the length of the tube.

DESCRIPTION OF THE DRAWINGS

The foregoing summary as well as the following description will bebetter understood when read in conjunction with the drawing FIGURE whichis a block flow diagram of a process for making a variable-wallthickness tube in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, a method 10 for manufacturing a variable wallthickness tube in accordance with the present invention is shown. Themethod 10 may be used to manufacture variable wall thickness tubes foruse in a variety of applications. For purposes of this description, themethod 10 will be described in the context of heat exchanger tubes, withthe understanding that the method of the present invention may be usedto manufacture tubes for use in a variety of applications.

The method 10 is designed to produce a tube having a substantiallyuniform outer diameter and a non-uniform inner diameter that fluctuatesalong the length of the tube. The uniform outer diameter and non-uniforminner diameter provide a tube wall that varies in thickness along thelength of the tube. The inner diameter of the tube fluctuates along thelength of the tube between a minimum inner diameter and a maximum innerdiameter. The tube wall has a maximum wall thickness, or T_(max), atthose sections where the inner diameter is at its minimum. Conversely,the tube wall has a minimum wall thickness, or T_(min), at thosesections where the inner diameter is at its maximum. In thisarrangement, the tube profile can be divided into a first group of wallsections having wall thickness T_(max), and a second group of wallsections having wall thickness T_(min). Wall sections having thicknessT_(min) provide substantial flexibility in the tube wall and permit thetube to flex. The presence of thinner wall sections also reduces theweight of the tube. Wall sections with wall thickness T_(max) are morerigid than sections having thickness T_(min) and substantially preventthe tube from buckling when the tube is flexed.

In addition to providing flexibility without buckling, the non-uniforminner diameter provides beneficial fluid flow and heat transfercharacteristics. In straight-walled tubes, pressurized fluid flow tendsto be more or less laminar, with fluid remaining generally in the samelocation relative to the tube cross-section as it passes through thetube. Fluid toward the outer circumference of the tube remains near theouter circumference, and fluid near the center of the tube remains nearthe center. Since heat passes through the tube wall, the fluid near theouter circumference of the tube has a higher rate of heat transfer thanthe fluid at the center axis, resulting in a temperature gradient thatdecreases outwardly from the center of the tube. This temperaturegradient often limits the overall transfer of heat from the fluid, sincethe higher temperature fluid at the center of the tube remainsinsulated. In tubes with non-uniform inner diameters, the varying innerdiameter facilitates turbulent flow characteristics. In turbulent flow,the fluid is continuously mixed, limiting the development of temperaturegradients within the fluid. The non-uniform inner diameter alsoincreases the effective surface area available for heat transfer.Moreover, the turbulent flow characteristics provided by the non-uniforminner diameter can provide a scouring effect and limit the accumulationof solids in the tube.

The method 10 can be applied in several ways to form a variable wallthickness tube, and the arrangement of sections having wall thicknessesT_(max) and T_(min) will vary depending on the desired properties of thetube. The inner contour of the tube can be formed with a variety ofgeometric configurations, including curved annular projections, flatannular projections, or a combination of curved and flat projections.The projections may be spaced apart at regular intervals, or varyingintervals.

The appropriate wall thickness of the tube is dependent on the selectedouter diameter of the tube and the physical properties of the materialselected in manufacturing the tube. The ratio of wall thickness tooutside diameter preferably varies from about 0.03 to 0.10. That is, theratio of wall thickness to outside diameter is preferably about 0.03where the tube has a minimum wall thickness, and about 0.10 where thetube has a maximum wall thickness. Tubes with lower or higher ratios mayalso be used with satisfactory results. The outer diameter of the tubemay be selected based on the particular application of the tube. Basedon the ratio stated above, a tube having an outside diameter of 0.1 inchpreferably has a minimum wall thickness of about 0.003 inches and amaximum wall thickness of about 0.01 inches. More preferably, a tubehaving an outside diameter of 0.1 inch has a maximum wall thickness ofabout 0.005 inches.

The relative spacing and dimensions of annular projections along theinner wall of the tube may vary considerably without departing from thescope of the invention. For instance, the axial width of the projectionsmay be a few hundredths of an inch, or several inches.

A shaped mandrel rod is used to form the variable wall-thickness tube.Tubing is drawn through a die and over the shaped mandrel rod to createthe variable wall thickness tube. In general, the inner contour of thetube is the inverse of the exterior contour of the shaped mandrel. Themandrel is preferably cylindrical with an outer diameter that fluctuatesbetween a minimum outer diameter and a maximum outer diameter. Themandrel may have a variety of shapes adapted to form a tube wall with afluctuating inner diameter. For example, to form a series of roundedgrooves on the inner surface of the tube, a mandrel is selected having aseries of rounded circumferential ridges that extend radially outwardlyfrom the mandrel. Preferably, the mandrel rod is sufficiently long toform multiple tubes in a single draw operation. In particular, themandrel rod preferably includes multiple segments, with each segmentdesigned to form one tube unit. Once the tubing is drawn over themandrel rod, individual tube units can be cut from the drawn tubing andtrimmed to finished lengths. The mandrel rod segments are spaced apartfrom one another so that each tube unit is separated by a shortdistance. In this arrangement, a small amount of excess tubing isprovided on each end of the tube units to allow the tube ends to betrimmed to a finished length.

Referring to the drawing, the method 10 is illustrated in block diagramform, with individual steps designated generally by reference numbers100-900. In step 100, a length of tubing, or “starting tube” isprepared. The tube may be formed from a variety of alloy materials,including but not limited to materials comprising stainless steel,titanium or cobalt-chrome alloy. The tubes may be prepared by bendingand welding strips of alloy. Strips having a thickness of between0.010-0.025 inches have been used successfully in forming tubing. Whereseamless tubing is required, the tubes may be prepared by gun drillinground bar stock. The starting tube diameter will vary depending on thedesired application. The method steps that follow have been applied tostarting tubes having an outer diameter of 0.25 inches.

Once the starting tube is formed, the outer diameter of the tube isreduced in step 200 by cold drawing the tube through a first die. Thefirst die is a round outer die configured to reduce the tube to anintermediate outer diameter. Typically, the tube is prepared for colddrawing by pointing the end of the tube and lubricating the outersurface of the tube. The end of the tube is pointed so that the outerdiameter of the tube end is slightly smaller than the diameter of thedraw die. In this way, the tube end can be gripped and fed easily intothe die. The outer surface of the starting tube is then coated with asuitable coating or lubricant to permit the tube to be passed smoothlythrough the die, and to prevent damage to the tube caused by frictionforces. Once the starting tube is prepared, the tube is cold drawnthrough a die to decrease the outer diameter to a predeterminedintermediate diameter. Following cold drawing, the tube is cleaned withan alkaline cleaner or other suitable cleaning solution to remove thecoating or lubricant on the tube.

Once the tube is cold drawn through the die and cleaned, the tube may beannealed, as shown in step 300. Annealing is done to reduce internalstresses created during cold drawing, soften the tube and providesufficient ductility in the tube material to permit further drawing, ifdesired. Preferably, the tube is annealed at a temperature of about1,900° F.-2,250° F. for about 1-2 minutes after drawing the tube.

If the intermediate outer diameter of the tube is not achieved aftercold drawing and annealing the tube, the cold drawing step 200 andannealing step 300 may be repeated. Once the intermediate outer diameteris achieved, the tube is cold drawn through a second die in step 400.The second die is a round outer die having a smaller diameter than thefirst die used in step 200. The tube is drawn through the second die andover a shaped mandrel to shape the inner surface of the tube. Prior todrawing, the tube may be pointed as necessary, and the inner and outersurface may be lubricated with a suitable coating or lubricant to permitthe tube to be passed smoothly through the die, and to prevent damage tothe tube caused by friction forces. The mandrel is positioned in thetube, and the tube and mandrel are fed through the second die. As thetube and mandrel pass through the second die, the tube wall flows into ashape conforming with the exterior contour of the mandrel.

When the tube and mandrel are passed through the second die, the tubewall flows into the spaces between the outer die and the mandrel to forman inner wall with a shape that is inverse to the exterior of themandrel. The drawn tube and mandrel combination are then processed toseparate the mandrel from the shaped tube in step 500. In the separatingstep 500, the diameter of the tube wall is expanded to provide clearancebetween the mandrel and inner tube wall, allowing the mandrel to beremoved from the tube. The extent of tube wall expansion varies, but istypically sufficient to increase the inner diameter of the tube so thatthe minimum inside diameter of the tube exceeds the maximum outerdiameter of the mandrel. One method to accomplish this is to run thetube and mandrel combination through a roll type tube straightener. Theflexing of the tube and mandrel in the straightening process increasesthe tube diameter, providing sufficient clearance between the innersurface of the tube and the outer surface of the mandrel to allowextraction or removal of the mandrel from the tube. After the tube andmandrel are separated, the tube is cleaned to remove the coating orlubricant from the inner and outer surfaces of the tube. If desired, thetube is then annealed in step 600 in the same general manner describedin step 300.

As the tube is drawn with the mandrel through the second die, the outersurface of the tube may or may not develop ridges or depressions inresponse to material flow over the surface of the mandrel. If ridges ordepressions develop, the tube may be passed through a finishing step toprovide a smooth exterior with a uniform outer diameter along the lengthof the tube. The exterior surface may be finished in a variety of ways.As shown in the drawing, the tube is sink drawn through a third die instep 700 to provide a smooth outer surface. Prior to sink drawing, thetube is pointed as necessary and lubricated. The tube is then passedthrough the third die. The third die is a round outer die with adiameter slightly smaller than the diameter of the second outer die. Thethird die is configured to reduce the tube to a desired finisheddiameter. The tube may be sink drawn in one or more passes to providethe desired finish. After the outer surface is finished and cleaned toremove any coating or lubricant, the tube may be annealed in step 800.The tube may be straightened as necessary and cut transversely todesired lengths in step 900.

In many cases, tube sections with thickness T_(min) are not rigid enoughto withstand compressive forces exerted on the tube by gripping toolsand other implements. If the ends of the finished tube have thicknessT_(min), the ends may buckle or pinch closed when being handled.Therefore, it may be desirable to shape the inner surface of the tube sothat the ends of the tube have a larger wall thickness that extends fora short length along each end of the tube. In this way, the ends of thetube will be relatively rigid and resistant to buckling when handled bytools.

The tube may be manufactured in different lengths, and the length of theworkpiece during manufacturing will vary depending on many factors,including the desired application of the end product, and themanufacturing equipment used to form the tube. If circumstances permit,the tube is preferably cold drawn in long lengths to form multiple tubeson one length of tubing. By processing tubes in longer lengths, themanufacturer reduces the number of times that the tubing must bepointed, lubricated, drawn and annealed. This can significantly reducethe time and effort expended in producing the desired quantity offinished tubes.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation. There is no intention in the use ofsuch terms and expressions of excluding any equivalents of the featuresshown and described or portions thereof. It is recognized, therefore,that various modifications are possible within the scope and spirit ofthe invention. Accordingly, the invention incorporates variations thatfall within the scope of the following claims.

1. A method for manufacturing a variable-wall thickness tube, saidmethod comprising the steps of: A. forming a generally cylindrical tube;B. drawing the tube through a first die to reduce the outer diameter ofthe tube to an intermediate outer diameter; C. placing the tube over amandrel having an outer diameter that fluctuates along the length of themandrel between a first diameter and a second diameter; D. drawing thetube and mandrel through a second die to form a tube wall having athickness that fluctuates along the length of the tube between a firstwall thickness and a second wall thickness; and E. removing the mandrelfrom the tube.
 2. The method of claim 1, comprising the step offinishing the outer surface of the tube so that the tube has asubstantially uniform outer finished diameter.
 3. The method of claim 2,wherein the step of finishing the outer surface comprises the step oftube sinking.
 4. The method of claim 2, comprising the steps ofstraightening and cutting the tube to a predetermined length after theouter surface finishing step.
 5. The method of claim 1, wherein the stepof forming the generally cylindrical tube comprises the step of bendinga metal strip having a pair of side edges to form a cylinder and weldingthe side edges together.
 6. The method of claim 1, wherein the step offorming the generally cylindrical tube comprises the step of drilling abore through the center of a piece of bar stock.
 7. The method of claim1, comprising the step of annealing the tube after the outer surfacefinishing step.
 8. The method of claim 1, wherein the step of removingthe mandrel from the tube comprises the step of running the mandrel andthe tube through a roll type tube straightener to provide clearancebetween the tube and the mandrel and allow extraction of the mandrelfrom the tubing.
 9. A method for manufacturing a plurality of variablewall thickness tubes, said method comprising the steps of: A. drawing alength of tubing through a first die; B. placing the tubing over amandrel rod comprising a variable diameter section and a uniformdiameter section abutting said variable diameter section; C. drawing thetubing and mandrel rod through a second die to form a first tubingsection and a second tubing section, said first tubing section having afluctuating inner diameter, and said second tubing section having auniform inner diameter; D. removing the mandrel from the tubing; and E.cutting the tubing transversely through each of said second tubingsections to form a plurality of tubes.
 10. The method of claim 9comprising the step of finishing the outer surface of the tubing so thatthe tubing has a substantially uniform outer diameter.
 11. The method ofclaim 10 comprising the step of removing the tubing from the mandrelafter drawing the tubing and mandrel through the second die and prior tothe finishing step.
 12. The method of claim 10, wherein the step offinishing the outer surface comprises the step of sink drawing thetubing.
 13. The method of claim 9, wherein the mandrel rod has a secondvariable diameter section abutting said uniform diameter section, and asecond uniform diameter section abutting the second variable diametersection.
 14. The method of claim 9, wherein the step of removing themandrel from the tubing comprises the step of running the mandrel andthe tubing through a roll type tube straightener to provide clearancebetween the tubing and the mandrel and allow extraction of the mandrelfrom the tubing.
 15. A method for manufacturing a plurality of variablewall thickness tubes, said method comprising the steps of: A. drawing apiece of tubing through a first die to reduce the outer diameter of thetubing to an intermediate outer diameter; B. placing the tubing over amandrel rod comprising: a plurality of first sections having a uniformrod diameter; and a plurality of second sections arranged alternatelywith said first sections, each of said second sections having a roddiameter that fluctuates between a first rod diameter and a second roddiameter; C. drawing the tubing and mandrel rod through a second die toform: a plurality of first tubing sections, each of said first tubingsections having an inner diameter that fluctuates between a first innerdiameter and a second inner diameter that is less than the first innerdiameter; and a plurality of second tubing sections, each of said secondtubing sections having a uniform inner diameter that is less than thefirst inner diameter of the first tubing sections; D. removing themandrel from the tubing; and E. cutting the tubing through each of saidsecond tubing sections to form a plurality of tubes.
 16. The method ofclaim 15 comprising the step of finishing the outer surface of thetubing so that the tubing has a substantially uniform outer diameter.17. A method for manufacturing a variable-wall thickness tube, saidmethod comprising the steps of: A. placing a generally cylindrical tubeover a mandrel having an outer diameter that fluctuates along the lengthof the mandrel between a first diameter and a second diameter; B.drawing the tube and mandrel through a die to form a tube wall having athickness that fluctuates along the length of the tube between a firstwall thickness and a second wall thickness; and C. removing the mandrelfrom the tube.
 18. The method of claim 17 comprising the step offinishing the outer surface of the tube so that the tube has asubstantially uniform outer finished diameter.